1. Field of the Invention
This invention relates to a novel method for producing known compounds useful as a plasticizer or lubricating oil. More particularly, this invention relates to a novel method for producing composite esters consisting of a dibasic acid, a diol, and a monofunctional alcohol component.
2. Description of the Prior Art
There have been known composite esters represented by a general formula: EQU R'O(COACOORO).sub.n COACOOR' (a)
wherein A is a straight chain or cyclic hydrocarbon residue of 0 to 6 carbon atoms with the proviso that A is a bond when the number of carbon atom is 0, R is a residual group of straight or branched chain diol or ester alcohol, and R' is a residual group of straight or branched chain alcohol of 4 to 10 carbon atoms and n is an integer of 1 to 6. For example U.S. Pat. No. 2,730,811 discloses a production of bis-compounds by stoichiometric reaction.
In Japanese Patent Application No. 151,688/1983, I have disclosed a method for producing predominantly a bis-compound by using a diester and a diol, and in Japanese Patent Application No. 135,272/1984, a polyvinyl chloride composition in which a composite ester containing an oligomer obtained by this method is used as a plasticizer.
It is known to obtain plasticizers or lubricating oils (hereinafter referred to as plasticizers, etc.) by conducting esterification reaction using a monobasic acid or a monovalent alcohol which is to become terminal groups in the production of a polyester from a dibasic acid and a diol, controlling the molecular weight of formed polyester to obtain various polyesters having different combinations of molecular weight as well as acid and alcohol components, and such plasticizers etc. are on the market. Plasticizer properties of these plasticizers products etc. however, are not always satisfactory.
According to the method of the above-mentioned United States Patent, it is extremely difficult to obtain bis compounds as a reaction product in a stoichiometric quantitative amount.
Namely, in the above-mentioned reaction, since there is no difference of reactivity between each acid and alcohol group of the compound, a mixture of oligomers wherein n is 1 to 6 is formed and a bis compound wherein n=1 is produced in an yield of only less than the half amount. Unlike the addition of a monofunctional alcohol in the above-mentioned production of a polyester from a dibasic acid and a diol, the present invention provides a method for producing the above-mentioned composite ester in which, starting from a dibasic acid represented by the formula HOOC--A--COOH wherein A is a straight chain or cyclic hydrocarbon of 0 to 6 carbon atoms, a diol represented by the formula HO--R--OH wherein R is a residual group of straight or branched chain diol or ether alcohol, and a monovalent alcohol represented by the formula R'OH wherein R' is a residual group of a straight or branched chain C.sub.4 -C.sub.10 alcohol, there is obtained, as an intermediate, a diester of a dibasic acid with a diol and a monovalent alcohol represented by the formula R'OCOACOOROH (hereinafter referred to as ester alcohol), from which a composite ester is produced by carrying out alcohol removing ester-interchange reaction between that diester (ester alcohol) and a diester represented by the formula R'OCOACOOR' (hereinafter referred to simply as diester). This is represented by the following reaction formula; ##STR1##
In the equilibrated second step of the formulae, the product composite ester (VI) can be obtained by removing R'OH.
It is well known for dehydrating esterification reaction to use an excess alcohol to produce an ester from an acid and an alcohol. In a system wherein a dibasic acid (I), a diol (II), and a monovalent alcohol (III) are mixed, the reaction product is obtained in a stoichiometric amount of the three components when the equivalent of the alcohols (diol and mono-alcohol) is equal to the equivalent of the acid. When the dehydrating esterification reaction is carried out in the presence of a dehydrating esterification catalyst, reaction rate of esterification accompanying formation of water becomes extremely slow at the late stages of the reaction. Accordingly, it is necessary to carry out esterification reaction in the presence of an excess amount of alcohols.
However, if an excess alcohol is added, ester interchange between the products concurrently occurs and once formed products including bis-compound (VI) are changed to higher molecular weight products. At the same time, the ester alcohol (IV) is formed as a by-product in an amount corresponding to the excess amount of the added alcohol. Although this ester alcohol (IV) has a lower boiling point compared with the bis-compound (VI) and its removal could be possible by distillation under reduced pressure in some cases, it is very difficult to completely remove it by distillation. When it remains in higher boiling point ester products, OH equivalent of the product is increased. Also, when it is added to a polyvinyl chloride as a plasticizer, a stabilizer therein acts as an ester interchange catalyst to cause ester interchange reaction, particularly at high temperatures, to form low boiling or volatile R'OH. This can be a cause of foaming and volatile material formation in use at high temperatures. Accordingly, removal of the ester alcohol is absolutely necessary.
On the other hand, when an ester mixture of a lower acid value is made by reaction in stoichiometric amounts followed by alkali-washing or the like to avoid unfavorable ester alcohol formation, there are problems in preparation such as increased raw material requirement per product due to the loss of unreacted material which is removed, as well as difficulty of separation of oil and water on account of the presence of alkali salt which acts to assist in emulsification, and thus it is difficult to attain low acid value.
The present invention has been completed on the basis of a discovery that a composite ester (VI) is produced when the above-mentioned dibasic acid (I), diol (II) and monovalent alcohol (III) are esterified in a proper mol ratio in the presence of a dehydrating ester catalyst, and then alcohol removing ester-interchange reaction is carried out using a titanium catalyst such as titanium tetrabutoxide, titanic acid ester, or a catalyst such as divalent tin compounds. Namely, I have discovered, when removing low boiling materials without removing catalyst after the use of a titanium catalyst, that alcohol removing reaction proceed. Thus, I have discovered that the ester interchange reaction proceeds easily by removing the produced alcohol with stirring and heating at 120.degree. to 200.degree. C. under reduced pressure.